In such free-running circuits, the switching elements must be supplied with drive power in time with the switching frequencies.
To this end, known arrangements use a separate current transformer, which is designed either as a saturable-current (toroidal-core) transformer or as a transformer with defined air gap (Hirschmann, W.: "Elektronikschaltungen" Siemens AG, 1982, pages 148 and 150).
The disadvantages of an arrangement of this type consist in that a separate, expensive wound part is required, the manufacturing tolerances of which at the same time have a decisive effect on the functional parameters of the overall arrangement.
Another possibility consists in fitting additional auxiliary windings to the resonant or current-limiting inductor which is already required (cf. L2 in FIG. 1) and applying the control signals thereby obtained to the gate or base inputs of the switching transistors via a phase-shifter network (P 41 29 430.0).
The disadvantage of a circuit arrangement of this type is that, at high DC voltage intermediate circuit voltages, the step-type component of the voltage across the resonant coil, caused by the switch-over of the half-bridge switches, is very strongly superimposed on the sinusoidal component of the voltage which is defined by the resonant current oscillation. As a result, the shape of the signal provided by the auxiliary windings resembles a square-wave function more than a sine function. The phase shifter proposed in the prior art (P 4129430.0) therefore operates only as a lag component, the capacitor of which can no longer be discharged during one RF period by-the sinusoidally decreasing secondary voltage fast enough to ensure reliable switching off of the half-bridge switches during an on-load half-cycle.